The present invention relates to a negative photosensitive lithographic printing plate. More particularly, the invention relates to a negative photosensitive lithographic printing plate suitable for drawing with a laser beam.
PS plates comprising a hydrophilic support and an oleophilic photosensitive resin layer formed thereon have hitherto been widely used as negative photosensitive lithographic printing plates. In general, a desired printing plate has been obtained from the PS plate by subjecting it to exposure to light (a real exposure) through a mask, i.e., a lith film, and then removing the nonimage areas by dissolution.
In recent years, the digitization technology in which image information is electronically processed, stored, and outputted with a computer is spreading widely. With this trend, various new techniques for image output which are suitable for the digitization technology have come to be practically used. As a result, it is desired to establish a computer-to-plate (CTP) technology in which a photosensitive resin layer is scanned with a highly directional light such as a laser beam according to digitized image information to thereby directly produce a printing plate without via the use of a lith film. An important technical subject therefore is to obtain a negative photosensitive lithographic printing plate suitable for use in that technology.
A negative photosensitive lithographic printing plate capable of such scanning exposure has been proposed which comprises a hydrophilic support and formed thereon an oleophilic photosensitive resin layer (hereinafter referred to also as xe2x80x9cphotosensitive layerxe2x80x9d) containing a photosensitive compound capable of generating an active species such as radicals or a Brxcfx86nstead acid, upon exposure to a laser beam. The negative photosensitive lithographic printing plate has already been put on the market. From this negative photosensitive lithographic printing plate can be obtained a negative lithographic printing plate by scanning it with a laser beam based on digital information to generate an active species, causing the photosensitive layer to undergo a physical or chemical change by the action of the active species to thereby insolubilize the exposed areas of the layer, and subsequently developing the layer.
In particular, a negative photosensitive lithographic printing plate which comprises a hydrophilic support and formed thereon a photopolymerization type photosensitive layer having a high photosensitization speed containing a photopolymerization initiator, an ethylenically unsaturated compound capable of undergoing addition polymerization, and a polymeric binder soluble in an alkaline developing solution and which optionally has a protective layer having oxygen barrier properties, attains excellent productivity, can be developed easily, and can give a printing plate having desirable printing performances because of its advantages of high resolution and satisfactory ink receptivity.
Organic polymers developable with an alkali have hitherto been used as the polymeric binders serving as components of photosensitive layers. Examples thereof include the methacrylic acid copolymers, acrylic acid copolymers, itaconic acid copolymers, crotonic acid copolymers, maleic acid copolymers, and partially esterified maleic acid copolymers shown in, e.g., Japanese Patent Laid-Open No. 44615/1984, Japanese Patent Publication Nos. 34327/1979, 12577/1983, and 25957/1979, and Japanese Patent Laid-Open Nos. 92723/1979, 53836/1984, and 71048/1984.
However, the negative photosensitive lithographic printing plates having a photosensitive layer containing such a conventional polymeric binder have had the following problem. When the speed of scanning is increased in order to further heighten productivity, the exposed areas come not to be sufficiently cured because the exposure light energy applied thereto per unit area becomes lower accordingly. As a result, the image areas are damaged during development by the alkali ingredient contained in the developing solution, so that high press life cannot be obtained. Consequently, a further improvement in productivity has been difficult.
On the other hand, photosensitive compositions containing a modified poly(vinyl alcohol) resin having radical-polymerizable pendant groups grafted to the polymer backbone are described in Japanese Patent Laid-Open No. 2000-181062 and International Patent Publication No. 2000-506282. However, these compositions are not intended to be used for producing lithographic printing plates. Furthermore, since these photosensitive compositions take advantage of the water solubility inherent in poly(vinyl alcohol) so as to be developed with water, they have had the following problem when used as a binder for lithographic printing plates. As the content of the radical-polymerizable groups incorporated, which are effective in improving press life, is increased, the content of hydroxyl groups, which impart developability, becomes low in proportion thereto, resulting in impaired developability. Namely, it has been difficult to attain both high press life and developability.
Accordingly, an object of the invention is to eliminate the above-described drawbacks of the conventional art and to provide a negative photosensitive lithographic printing plate, especially one suitable for drawing with a laser beam, from which a lithographic printing plate having sufficient press life can be obtained with high productivity without impairing developability even under such exposure conditions that the irradiation energy per unit area is low.
The present inventors made intensive investigations in order to accomplish the object. As a result, it has been found that when a modified poly(vinyl alcohol) resin having a radical-polymerizable group and an acid group is used as a polymeric binder serving as a component of a photosensitive layer, then the object is accomplished.
(1) A negative photosensitive lithographic printing plate comprising:
a support; and
a photosensitive layer containing:
a modified poly(vinyl alcohol) resin binder having a radical-polymerizable group and an acid group; and
at least one of a photo-polymerization initiator and a heat-polymerization initiator.
(2) The negative photosensitive lithographic printing plate as described in item (1), wherein the modified poly(vinyl alcohol) resin binder contains: at least one of repeating units represented by formulae (I) and (II); and at least one of repeating units represented by formula (III): 
wherein A and B each independently represents a radical-polymerizable group; X represents an acid group; R1, R2 and R3 each independently represents a substituted or unsubstituted hydrocarbon group having 1 to 30 carbon atoms, and R1, R2and R3 each has a valent of (m+1), (n+1) and (p+1) respectively; and m, n, and p each independently represents an integer of 1 to 5.
(3) The negative photosensitive lithographic printing plate as described in item (1), wherein the radical-polymerizable group has an addition-polymerizable unsaturated bond.
(4) The negative photosensitive lithographic printing plate as described in item (1), wherein the radical-polymerizable group has at least one selected from the group consisting of a (meth)acryloyl group, (meth)acrylamide group, allyl group and styrene structure.
(5) The negative photosensitive lithographic printing plate as described in item (1), wherein the acid group has an acid dissociation constant: pKa of 7 or lower.
(6) The negative photosensitive lithographic printing plate as described in item (1), wherein the acid group is selected from the group consisting of xe2x80x94COOH, xe2x80x94SO3H, xe2x80x94OSO3H, xe2x80x94PO3H2, xe2x80x94OPO3H2, xe2x80x94CONHSO2xe2x80x94 and xe2x80x94SO2NHSO2xe2x80x94.
(7) The negative photosensitive lithographic printing plate as described in item (1), wherein the acid group is xe2x80x94COOH.
(8) The negative photosensitive lithographic printing plate as described in item (2), wherein the repeating unit represented by formula (III) is represented by formula (IV): 
wherein R3 represents a substituted or unsubstituted hydrocarbon group having 1 to 30 carbon atoms, and R3 has a valent of (p+1); and p represents an integer of 1 to 5.
(9) The negative photosensitive lithographic printing plate as described in item (8), wherein R3 in the formula (IV) contains at least one of an aliphatic ring structure and an aromatic ring structure.
(10) The negative photosensitive lithographic printing plate as described in item (8), wherein R3 in the formula (IV) contains an aliphatic ring structure.
(11) The negative photosensitive lithographic printing plate as described in item (2), wherein the modified poly(vinyl alcohol) resin binder contains:
i) at least one of the repeating units represented by formulae (I) and (II) in an amount of from 1 to 99% by mole; and
ii) at least one of repeating units represented by formula (III) in an amount of from 1 to 70% by mole,
in which the sum of the repeating unit i) and the repeating unit ii) is 2 to 100% by mole.
(12) The negative photosensitive lithographic printing plate as described in item (1), wherein the photosensitive layer further contains a compound having at least one ethylenically unsaturated bond capable of undergoing an addition polymerization.
One of the features of the negative (negative working) photosensitive lithographic printing plate of the invention resides in that a modified poly(vinyl alcohol) resin having a radical-polymerizable group and an acid group is used as a polymeric binder. Due to the use of this resin, the negative photosensitive lithographic printing plate of the invention can give a printing plate having sufficient press life with high productivity without impairing developability even under such exposure conditions that the irradiation energy per unit area is low.
Although the reasons for this effect of the invention have not been elucidated, the following may be thought. Besides having high mechanical strength, the modified poly(vinyl alcohol) resin is more apt to be crosslinked to cure in exposed areas due to the incorporation of radical-polymerizable groups. As a result, sufficient press life can be obtained even in regions exposed at a low energy. Furthermore, due to the acid groups incorporated simultaneously, the unexposed areas have excellent removability by alkali development. Thus, an excellent printing plate which combines excellent productivity and press life can be obtained without impairing developability.